[#877] neofs-node/config: allow to provide default values

In case we have multiple sections with similar structure (e.g. shards)
having defaults in a single place is easier to work with.

Signed-off-by: Evgenii Stratonikov <evgeniy@nspcc.ru>

cmd/neofs-node/config/calls.go

@@ -15,9 +15,19 @@ func (x *Config) Sub(name string) *Config {
 
 	copy(path, x.path)
 
+	var defaultPath []string
+	if x.defaultPath != nil {
+		ln := len(x.defaultPath)
+		defaultPath = make([]string, ln, ln+1)
+		copy(defaultPath, x.defaultPath)
+	}
+
+	copy(path, x.path)
+
 	return &Config{
-		v:    x.v,
-		path: append(path, name),
+		v:           x.v,
+		path:        append(path, name),
+		defaultPath: append(defaultPath, name),
 	}
 }
 
@@ -30,5 +40,18 @@ func (x *Config) Sub(name string) *Config {
 //
 // Returns nil if config is nil.
 func (x *Config) Value(name string) interface{} {
-	return x.v.Get(strings.Join(append(x.path, name), separator))
+	value := x.v.Get(strings.Join(append(x.path, name), separator))
+	if value != nil || x.defaultPath == nil {
+		return value
+	}
+	return x.v.Get(strings.Join(append(x.defaultPath, name), separator))
+}
+
+// SetDefault sets fallback config for missing values.
+//
+// It supports only one level of nesting and is intended to be used
+// to provide default values.
+func (x *Config) SetDefault(from *Config) {
+	x.defaultPath = make([]string, len(from.path))
+	copy(x.defaultPath, from.path)
 }

cmd/neofs-node/config/calls_test.go

@@ -60,3 +60,20 @@ func TestConfig_SubValue(t *testing.T) {
 		require.Equal(t, "val1", sub.Value("key"))
 	})
 }
+
+func TestConfig_SetDefault(t *testing.T) {
+	configtest.ForEachFileType("test/config", func(c *config.Config) {
+		c = c.Sub("with_default")
+		s := c.Sub("custom")
+		s.SetDefault(c.Sub("default"))
+
+		require.Equal(t, int64(42), config.Int(s, "missing"))
+		require.Equal(t, "b", config.String(s, "overridden"))
+		require.Equal(t, false, config.Bool(s, "overridden_with_default"))
+
+		// Default can be set only once.
+		s = s.Sub("sub")
+		require.Equal(t, int64(123), config.Int(s, "missing"))
+		require.Equal(t, "y", config.String(s, "overridden"))
+	})
+}

cmd/neofs-node/config/config.go

@@ -17,7 +17,8 @@ import (
 type Config struct {
 	v *viper.Viper
 
-	path []string
+	defaultPath []string
+	path        []string
 }
 
 const separator = "."

cmd/neofs-node/config/test/config.json

@@ -56,5 +56,24 @@
     "size_tb": "5 TB",
     "size_bytes": "2048b",
     "size_bytes_no_suffix": 123456
+  },
+
+  "with_default": {
+    "default": {
+      "sub": {
+        "missing": 123,
+        "overridden": "x"
+      },
+      "missing": 42,
+      "overridden": "a",
+      "overridden_with_default": true
+    },
+    "custom": {
+      "sub": {
+        "overridden": "y"
+      },
+      "overridden": "b",
+      "overridden_with_default": false
+    }
   }
 }

cmd/neofs-node/config/test/config.yaml

@@ -49,3 +49,17 @@ sizes:
   size_tb: 5 TB
   size_bytes: 2048b
   size_bytes_no_suffix: 123456
+
+with_default:
+  default:
+    sub:
+      missing: 123
+      overridden: "x"
+    missing: 42
+    overridden: "a"
+    overridden_with_default: true
+  custom:
+    sub:
+      overridden: "y"
+    overridden: "b"
+    overridden_with_default: false